Complete each sentence choosing the correct option
The massive movement of butterflies begins its southern migration in September and October. Eastern and northeastern populations of monarchs, originating in southern Canada and the United States, travel to overwintering sites in central Mexico. They arrive at their roosting sites in November and remain there during the winter months. They then begin their northern migration in March.
No individual butterfly completes the entire round trip and a total of four generations are involved in the annual cycle. Female monarchs lay eggs for a subsequent generation during the northward migration.
Similarly, the western populations of monarchs migrate annually from regions west of the Rocky Mountains to overwintering sites on the coast of California.